This topic describes how to synchronize data from a user-created MySQL database connected
over Express Connect, VPN Gateway, or Smart Access Gateway to an ApsaraDB RDS for
MySQL instance by using Data Transmission Service (DTS).

Notes

DTS uses read and write resources of the source and destination databases during initial
full data synchronization. This may increase the database load. If the database performance
is unfavorable, the specification is low, or the data volume is large, database services
may become unavailable. For example, DTS occupies a large amount of read and write
resources in the following cases: a large number of slow SQL queries are performed
on the source database, the tables have no primary keys, or a deadlock occurs in the
destination database. Before synchronizing data, you must evaluate the performance
of the source and destination databases. We recommend that you synchronize data during
off-peak hours. For example, you can synchronize data when the CPU usage of the source
and destination databases is less than 30%.

If you have selected one or more tables (not a database) for synchronization, do not
use gh-ost or pt-online-schema-change to modify the tables during data synchronization.
Otherwise, data synchronization may fail.

Notice To avoid synchronization failure, you can use Data Management (DMS) to perform online
DDL schema changes during data synchronization. For more information, see Change the table schema without locking.

You cannot synchronize data to an ApsaraDB RDS for MySQL instance that resides in
Zone A of the China (Hong Kong) region.

The destination ApsaraDB RDS for MySQL instance must have an internal endpoint.

If the source database does not have primary keys or UNIQUE constraints, and fields
are not required to be unique, duplicate data may exist in the destination database.

During initial full data synchronization, concurrent INSERT operations cause fragmentation
in the tables of the destination instance. After initial full data synchronization,
the tablespace of the destination instance is larger than that of the source instance.

Limits

Incompatibility of triggers

If the object you want to synchronize is a database and the database contains a trigger
that updates the synchronized table, the synchronized data may be inconsistent. For
example, the source database contains Table A and Table B. If a data record is inserted
into Table A, a trigger inserts a data record into Table B. In this case, after an
INSERT operation is performed on Table A in the source instance, the data in Table
B becomes inconsistent between the source and destination instances.

To avoid this situation, you must delete the trigger that is synchronized to the destination
instance and select Table B as the object to be synchronized. For more information,
see Configure synchronization when triggers exist.

Limits on RENAME TABLE operations

RENAME TABLE operations may cause data inconsistency between the source and destination
databases. For example, if only Table A needs to be synchronized and it is renamed
Table B, Table B cannot be synchronized to the destination database. To avoid this
situation, you can select the database to which Table A and Table B belong as the
object when configuring the data synchronization task.

The region of the source instance. The region is the same as the region that you selected
when you purchased the data synchronization instance. You cannot change the value
of this parameter.

Peer VPC

Select the ID of the VPC that is connected to the user-created MySQL database.

Database Type

The value of this parameter is set to MySQL and cannot be changed.

IP Address

Enter the server IP address of the user-created MySQL database.

Port Number

Enter the service port number of the user-created MySQL database. The default port
number is 3306.

Database Account

Enter the account for the user-created MySQL database. The account must have the REPLICATION
SLAVE permission, the REPLICATION CLIENT permission, the SHOW VIEW permission, and
the permission to perform SELECT operations on the required objects.

Database Password

Enter the password for the database account.

Destination Instance Details

Instance Type

Select RDS Instance.

Instance Region

The region of the destination instance. The region is the same as the region that
you selected when you purchased the data synchronization instance. You cannot change
the value of this parameter.

Instance ID

Select the ID of the destination RDS instance.

Database Account

Enter the database account for the destination RDS instance.

Note If the database engine of the destination RDS instance is MySQL 5.5 or MySQL 5.6, you do not need to configure the database account or database password.

Note The Encryption parameter is available only in mainland China and Hong Kong(China).

In the lower-right corner of the page, click Set Whitelist and Next.

Configure the synchronization policy and objects.

Parameter

Description

Processing Mode In Existed Target Table

Pre-check and Intercept: checks whether the destination database contains tables that have the same names
as tables in the source database. If the destination database does not contain tables
that have the same names as tables in the source database, the precheck is passed.
Otherwise, an error is returned during precheck and the data synchronization task
cannot be started.

Ignore: skips the precheck for identical table names in the source and destination databases.

Warning If you select Ignore, data consistency is not guaranteed and your business may be exposed to potential
risks.

If the source and destination databases have the same schema, and the primary key
of a record in the destination database is the same as that in the source database,
the record remains unchanged during initial data synchronization. However, the record
is overwritten during incremental data synchronization.

If the source and destination databases have different schemas, initial data synchronization
may fail. In this case, only some columns are synchronized or the data synchronization
task fails.

Objects to be synchronized

Select objects from the Available section and click the icon to move the objects to the Selected section.

You can select tables and databases as the objects to be synchronized.

Note

If you select a database as the object to be synchronized, all schema changes in the
database are synchronized to the destination database.

After an object is synchronized to the destination database, the name of the object
remains unchanged. You can change the name of an object in the destination instance
by using the object name mapping feature provided by DTS. For more information about
how to use this feature, see Specify the name of an object in the destination instance.

In the lower-right corner of the page, click Next.

Configure initial synchronization.

During initial synchronization, DTS synchronizes the schemas and data of the required
objects from the source instance to the destination instance. The schemas and data
are the basis for subsequent incremental synchronization.

Initial synchronization includes initial schema synchronization and initial full data
synchronization. You must select both Initial Schema Synchronization and Initial Full Data Synchronization in most cases.

In the lower-right corner of the page, click Precheck.

Note

Before you can start the data synchronization task, a precheck is performed. You can
start the data synchronization task only after the task passes the precheck.

If the task fails to pass the precheck, click the icon next to each failed item to view details. Troubleshoot the issues based on the
causes and run the precheck again.

Close the Precheck dialog box after the following message is displayed: The precheck is passed.

Wait until the initial synchronization is complete and the data synchronization task
is in the Synchronizing state.

You can view the status of the data synchronization task on the Data Synchronization page.